The present invention generally relates to mobile networks and, more particularly, to a system and method to determine link latency during the re-registration process of mobile IP enabled devices.
In traditional Internet Protocol (IP) routing, IP addresses represent the location of specific network nodes within a network topology, allowing packet delivery. To receive packets under this scheme, routing mechanisms assume that each network node maintains the same point of attachment to the network topology of the Internet. Furthermore, routing mechanisms assume that each network node has one associated IP address that uniquely identifies the network node's point of attachment. When a network node dispatches a packet intended for a recipient device, intermediate Internet routers receive the packet and analyze the subnet prefix of its IP address to determine the network of the recipient device. Each intermediate router forwards the packet to the ascertained next closest router. Finally, when the packet arrives at the route on the device's subnet, the router analyzes the remaining bits in the IP address to identify the specific recipient device and completes delivery.
If, however, a device connects to the Internet via a subnet other than the subnet associated with the IP address of the device, traditional Internet routing will not deliver data destined for the device to the subnet on which it is attached. Therefore, traditional IP routing does not provide sufficient means for communication to mobile device users traveling between subnets.
Mobile IP enhances traditional IP by adding mechanisms for forwarding Internet traffic to mobile nodes or mobile stations; i.e., mobile devices or wireless communication devices having, for example, a transmitter and a receiver. Typically, mobile devices include, but are not limited to, cellular telephones, handheld devices, laptop computers and routers. Mobile IP assigns each mobile device a traditional home IP address, which dictates its topologically correct “home” network. Such mobile devices also maintain an associated care-of address (COA) that identifies the location of the mobile device as it changes its point of attachment. Each time a user moves the mobile device to a different network, the mobile device acquires a new COA. A home agent (HA) on the home network associates each mobile's permanent address with its current COA.
Versions of Mobile IP include Mobile IPv4, Mobile IPv6, and HMIPv6. In Mobile IPv4, traffic for the mobile device is sent to the home network but is intercepted by the HA and forwarded via tunneling to the appropriate COA. Foreign agents (FA) may act as intermediary forwarding devices between the mobile device and the HA. In certain embodiments, packet data serving nodes (PDSNs) provide the primary wireless mobile data access to, for example, the Internet or intranets. Mobile IPv6 minimizes tunneling and includes mechanisms that make FAs unnecessary. HMIPv6 reduces the amount of signaling required to improve handoff speed for mobile connections by replacing FAs with Mobility Anchor Points (MAPs), which can be located anywhere in a hierarchy of routers.
Registration is a critical function in Mobile IP. Each time the mobile device changes its COA, it registers with the HA by forwarding a binding update to the HA via Internet Control Message Protocol (ICMP) in IPv6 networks, or by sending a Registration Request via the FA in IPv4 networks. Mobile IP registration provides a means for the mobile device to communicate its current location to the HA. Each registration has an associated lifetime for which the HA and FA (if applicable) will provide forwarding services.
In versions of Mobile IP using FAs, PDSNs, or MAPs (collectively, intermediary devices), the mobile device sends a registration request (RRQ) to the intermediary device to begin registration. The intermediary device processes the RRQ and relays it to the HA. Upon receipt of the RRQ, the HA processes the request and sends a registration reply (RRP) granting or denying the request to the intermediary device. In turn, the respective intermediary device processes the RRP and relays the reply to the mobile device, thus notifying it of the disposition of its request. In instances where the mobile device registers a co-located COA, the mobile device forwards the RRQ directly to the HA. The HA responds with a corresponding RRP directly to the mobile device.
Networks typically implement relatively short registration lifetimes to minimize the impact of zombie Mobile IP registrations on HA and FA/MAP resources. Therefore, to maintain connectivity, as well as accurate COA and reachability information, a mobile device requests re-registration to the HA via RRQs, sent in-band with data, often during data transfer. The mobile device then awaits receipt of an RRP from the HA corresponding to the RRQ. To address the possibility of loss of the RRQ or RRP messages, the mobile device typically sets a retry timer upon dispatch of an RRQ. If the mobile device receives a valid RRP prior to expiration of the time on the retry timer (RRQ timeout), re-registration is completed. If, however, the retry time expires prior to receipt of the corresponding RRP, the mobile device dispatches a subsequent RRQ. Unnecessary re-registration requests (RRQ retransmissions) may be sent when re-registration occurs during a data transfer since buffering and other factors may increase the latency of the re-registration reply beyond the RRQ timeout.
The Internet Official Protocol Standards (STD 1), Request For Comments (RFC) 2002, Section 3.6.3, IBM (1996) suggests a simple geometric backoff algorithm comprising the steps of sending a new RRQ after a specified retransmission timeout (minimum one second), and increasing the retransmission timeout (the backoff period), at a minimum, by a multiplier of two. Under this formula, the timeout eventually increases to a value greater than the time required for the RRP to arrive after the RRQ has been sent; i.e., the round trip time (RTT).
Despite the backoff algorithm's attempt to regulate retransmission of re-registration requests, if the mobile device receives a valid RRP to a predecessor request after dispatch of a successor request, then the mobile device is obligated to discard the reply, delaying completion of mobile node re-registration, increasing traffic on the network, and adversely loading the network infrastructure.
It is apparent that a need exists for an improved system and method for timing request retransmissions that effect re-registration while minimizing unnecessary transmissions. There is also a need for a system and method to enhance mobile device communications, minimize network loads, and optimize network traffic levels by means of optimized Mobile IP re-registration.